Spark-ignition internal combustion engines, i.e. Otto-cycle engines, are equipped with various exhaust gas aftertreatment systems in order to reduce the pollutant emissions. Even without additional measures, oxidation of the unburned hydrocarbons (HC) and of carbon monoxide (CO) duly takes place during the expansion and discharge of the cylinder charge at a sufficiently high temperature level and in the presence of sufficiently large oxygen quantities. However, due to the exhaust gas temperature which decreases rapidly in the downstream direction, and the consequently rapidly decreasing rate of reaction, said reactions are quickly halted.
For these reasons, use is made of catalytic reactors which, through the use of catalytic materials which increase the rate of certain reactions, ensure oxidation of HC and CO even at low temperatures. If nitrogen oxides are additionally to be reduced, this may be achieved through the use of a three-way catalytic converter, which, however, for this purpose utilizes stoichiometric operation (λ≈1) of the Otto-cycle engine within narrow limits Here, the nitrogen oxides NOx are reduced by means of the non-oxidized exhaust gas components which are present, specifically the carbon monoxides and the unburned hydrocarbons, wherein said exhaust gas components are oxidized at the same time.
In the case of internal combustion engines which are operated with a high excess of air, i.e., for example, Otto-cycle engines operating in the lean mode, but also direct injection Otto-cycle engines, the nitrogen oxides in the exhaust gas in principle cannot be reduced, i.e. because of the lack of reducing agents. For effective exhaust gas aftertreatment, the working methods described above would require the use of exhaust gas aftertreatment systems which are used in diesel engines which, in principle, are operated with a high excess of air. In order to oxidize the unburned hydrocarbons (HC) and carbon monoxide (CO), oxidation catalytic converters would have to be provided in the exhaust system. In order to reduce the nitrogen oxides, use could be made of selective catalytic converters, in which reducing agent is introduced in a targeted manner into the exhaust gas in order to selectively reduce the nitrogen oxides. The nitrogen oxide emissions could also be reduced with a nitrogen-oxide storage catalytic converter, in which the nitrogen oxides are initially absorbed in the catalytic converter during a lean mode of the internal combustion engine, i.e. are collected and stored, in order then to be reduced when oxygen is deficient during a regeneration phase.
Particle emissions were originally considered only to be a problem in diesel engines.
However, these soot emissions even of spark-ignition internal combustion engines have increasingly become the focal point of the legislature.
In order to keep to future limit values for pollutant emissions, in particular soot emissions, additional measures are therefore necessary, requiring spark-ignition internal combustion engines to be equipped with a particle filter.
“Regenerative particle filters” are already used in diesel engines to minimize the emission of soot particles. In this case, the soot particles are filtered out of the exhaust gas, stored and intermittently burned during regeneration of the filter. In order to oxidize the soot in the filter, oxygen or an excess of air is required in the exhaust gas, and this can be achieved, for example, by a superstoichiometric operation (λ>1) of the internal combustion engine.
Diesel engines and spark-ignition internal combustion engines differ considerably in respect of the working methods. In contrast to diesel engines which, in principle, are operated with a high excess of air (λ>>1), Otto-cycle engines are generally equipped with a three-way catalytic converter which—as explained above—utilizes stoichiometric operation within narrow limits In the case of the Otto-cycle engine, the use of a particle filter therefore requires a concept with which the oxygen required for regenerating the filter is provided, whereas, in the case of the diesel engine, an excess of air is in any case present in the exhaust gas because of the working method.
The high temperatures required for regenerating the particle filter, for example TReg≈550° C., when catalytic support is not present, are sufficiently frequently achieved in a spark-ignition internal combustion engine, even during normal operation, because of the high exhaust gas temperatures in comparison to the diesel engine.
Nevertheless, the filter has also to be able to be regenerated in a targeted manner if the current loading of the filter requires this. This is because, depending on the driving behavior of the particular driver, i.e. the manner in which the internal combustion engine is operated, it is no longer possible to assume that the conditions required for regeneration are readily achieved sufficiently frequently during operation, i.e. without assistance. For example, in the case of vehicles which are used only for short distances and which require a large number of cold starts, the filter may be critically loaded, since the conditions required for regeneration do not occur during operation. Additional measures therefore have to be resorted to in order to ensure regeneration of the filter.
The inventors herein have recognized the issues with the above approaches and herein offer a method to at least partly address them. In one embodiment, a method for operating a spark-ignition internal combustion engine having a particle filter for collecting and burning soot particles in exhaust gas, comprises, in order to initiate regeneration of the particle filter, increasing a particle filter temperature Tfilter to such an extent that Tfilter≧TReg, wherein TReg is a predefinable minimum regeneration temperature, and operating the internal combustion engine superstoichiometrically (λ>1).
According to the disclosure, the filter temperature Tfilter is raised in a targeted manner for the purpose of cleaning the filter so as to provide the conditions for regeneration of the filter. In addition, the oxygen required for the oxidation of the particles collected in the filter is provided by the internal combustion engine being operated superstoichiometrically (λ>1), i.e. also being transferred into the superstoichiometric mode if the internal combustion engine was operated substoichiometrically or stoichiometrically previously.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description.
It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.